The present invention relates to a magnetically graduated steel bar, and more particularly a magnetically graduated steel rod to be used with a positioning cylinder in the mechanical and electronic industry.
The position of positioning cylinders has conventionally been detected with sensors attached to the cylinders, such as rotary encoders.
FIG. 1 is a schematic diagram showing a layout for controlling the position of a piston rod by means of a conventional rotary encoder. A cylinder 10 is equipped with a piston rod 12, which is fitted with a position measuring roller 14. The rotational number of the roller 14 and other positioning data are sent to a position measuring unit 16 and the position of the rod 12 is determined by sensing the deviation of the rotational angle from a reference value. The unit 16 sends a control signal to a hydraulic drive circuit for the rod 12, and its position is corrected by opening either valve 20 or 22.
However, this method has various disadvantages including insufficient precision and lack of ruggedness. To solve these problems, Unexamined Published Japanese Patent Application Nos. 16309/1982, 83620/1987, etc. have proposed magnetic scales in the form of a piston rod on a cylinder that is graduated with respect to magnetism and combined with a magnetic sensor to detect the position of the rod itself. In fact, however, the magnetic scale disclosed in Unexamined Published Japanese Patent Application No. 16309/1982 has such small differences in magnetism in the graduated portion that a special design feature is necessary for the detecting unit; and, the precision of detection has not been very high. The magnetic scale described in Unexamined Published Japanese Patent Application No. 83620/1987 is characterized in that it should contain at least 10 vol % of martensite in order to insure its performance as a scale (the steel composition is not limited in any particular way). For practical applications, the martensite content is limited to 40% or less (11-26% in the examples) in order to prevent cracking. The highest tensile strength that can be attained in the examples is about 120 kgf/mm.sup.2. The patent makes no mention of fatigue characteristics which are important properties of steel bars.
Piston rods for use with cylinders are conventionally made of steel bars that are prepared by quenching and tempering "structural steels" which are typified by S45C. Magnetic scales in which a ferromagnetic martensite is combined with a nonmagnetic austenite have heretofore been proposed as magnetically graduated rods for use with positioning cylinders in the mechanical and electronic industry. However, those magnetic scales have had problems with respect to mechanical properties and the precision of the scale.
More specifically, steel bars that are to be used as piston rods on cylinders and which are graduated magnetically have been proposed as an alternative to the conventional sensors, but they have had the following disadvantages:
(1) Emphasis has been placed solely on improving scale characteristics, and the mechanical properties of the scales are by no means better than those of conventional scales; PA1 (2) The scales have unsatisfactory fatigue characteristics, which are extremely important mechanical properties; and PA1 (3) The scale characteristics are by no means satisfactory.
Furthermore, the use of such steel bars in heavy and varying load applications has increased, so their operating environments have become more severe. Specific applications include actuators for construction machines and large industrial machines used in high,load environments, as well as hydraulic units for vehicle suspensions subjected to high-repetition, varying loads.